1. Field of the Invention
The present invention relates to a switching power source circuit, particularly, to a switching power source which is useful in case of reducing power loss in a current resonance type switching power source.
2. Description of Related Art
FIG. 4 shows a principle diagram of a switching power source circuit constructed by a half-bridge circuit. Reference symbols Q1 and Q2 denote switching elements composed of MOSFETs connected serially. A reference symbol T denotes an isolation transformer to transfer switching power on a primary side to a secondary side.
IC denotes a power source control circuit to alternately turn on/off the switching elements Q1 and Q2 and, normally, is constructed so as to enable a switching frequency of the switching elements Q1 and Q2 to be varied while comparing an output voltage V0 with a reference voltage by voltage detecting means (not shown) and controls so as to set the output voltage V0 to a constant voltage.
The output of the switching elements Q1 and Q2 is supplied to a primary winding L1 of the isolation transformer T and a resonance capacitor C1. If the switching elements Q1 and Q2 are alternately turned on/off, the primary winding L1 of the transformer is driven by a current charging/discharging the resonance capacitor C1 resonating to a leakage inductance of the transformer T.
If, at a timing when a current flowing to the switching elements is set to 0, a switching operation of the switching elements is executed by the resonance current, a loss current of the switching elements can be decreased.
Diodes D1 and D2 of full-wave rectification and a smoothing capacitor C0 are connected to the secondary winding L2 of the isolation transformer T and the DC output voltage V0 is obtained.
However, in such a switching power source circuit, a voltage V1 and a current I1 as shown in FIG. 5 are caused by turn on/off of the switching elements Q1 and Q2. At the time of turn-on of the switching element Q1, a current flowing to the transistor starts to flow from the minus direction at inclination determined by a resonance condition, and a zero-cross is realized. As for a current I, as shown in FIG. 5, a resonance current IR corresponding to a generated resonance circuit is multiplexed to a current I1 and has the same magnitude as the current I1 used to transfer power. Therefore, the peak current value is increased.
Thus, there is a problem in which a power loss of the product of a saturation voltage and the current at the switching elements Q1 and Q2 is increased.
Particularly, with respect to the conventional current resonance type switching power source, a leakage inductance of the isolation transformer T is used so as to generate a resonance current and a resonance frequency is set by the leakage inductance and the resonance capacitor C1, so that it is difficult to design the isolation transformer T. Additionally, to generate a leakage magnetic flux to the isolation transformer T, for instance, it is designed so that the primary winding L1 and the secondary winding L2 are loosely coupled. Therefore, there arises a problem in which conversion efficiency of a power is lowered and a leakage magnetic field is caused.